Yeah, this is a great example. Darwin is such a cool figure in the history of science in part because--in contrast to other "Great Man" figures like Newton or Einstein--he didn't really come up with any novel methodological tools or earthshaking new ideas about the structure of reality. Everything Darwin said in The Origin of Species was out there already, and much of it had already even been proposed by other people (including his own grandfather Erasmus Darwin). His contribution was just systematizing all that stuff, providing a unified narrative, and explicitly thinking through the immediate consequences of a pile of things that was already known. People already knew (very roughly) about heritability of traits (though the mechanism would prove elusive for a long time after Darwin). They also already knew about environmental selection and competition for resources, and knew (or at least strongly suspected by that point) that the Earth was very, very old. If you just put all those facts next to each other, the consequence--that descent with modification can and does explain the diversity of life we see all around us--almost just pops out by itself, and once you start thinking that way lots of other stuff in biology suddenly clicks into place within the explanatory framework. There's that famous quote from Theodosius Dobzhansky that "nothing in biology makes sense except in light of evolution," and while that's a bit of an exaggeration, it's very close to being true. Evolution is the paradigm case of an explanatory theory: it's obvious in retrospect in large part because so much of what was before just a "pile of sundry facts" suddenly makes perfect sense as soon as you start looking at it through that explanatory lens. It always makes me wonder what else we're missing that might seem just as glaringly obvious to people a century from now.

For what it's worth, I think Marx's work is in pretty much the same boat: it's a theory whose power comes from providing a framework for making sense of a bunch of bits and pieces of data about the world that, in the absence of the theory, don't seem to quite fit together. That there are problems in the details of how the originators of each theory first expressed it (which there are in both cases) doesn't really matter much, because the insight is a way of looking at the world more than it is the precise articulation of this or that mechanism or phenomenon.